Tillie's character in the play The Effect of Gamma Rays on Man-in-the-Moon Marigolds has a two-fold role. The first, is to serve as a foil (contrast) of her mother, Beatrice in terms of her outlook on life, her hunger to succeed, and her resilience in times of difficulty. The second role is to provide the only glimmer of hope in the lives of the Hunsdorfer family; a dysfunctional family of three (and an additional, elderly female tenant) whose leader, Beatrice, seems to be leading them to despair and chaos.

Tillie's traits are found everywhere in the story. For example, the reader knows that Tillie's appearance is awkward because of the many instances in which Ruth criticizes the way that she dresses, her hair, and her likelihood to be "laughed at". Similarly, Beatrice gives hints as to Tillie's awkward looks when she says

You know, one day maybe you will be pretty. You'll have some nice features, when that hair revives and you do some tricks with make-up.

However, this is not the reason why she is an outcast in school, necessarily. Even though poor Tillie is neglected at home and has to wear whatever she finds, her behavior may come from way before her birth. This is because, as Ruth and Beatrice explain in separate occasions, Beatrice, herself, used to go to the same school that Tillie and Ruth attend. It is further implied that Beatrice was also quite awkward and laughed at when she was going to school and, just like Tillie, she too was an outcast. This being said, it is highly possible that Tillie's behavior is acquired from her mother. This is noteworthy, since it shows how Tillie is not only a foil of her mother, but she is also an extension of her; something that Beatrice obviously recognizes, and represses.

Tillie's ability to see beauty and goodness goes hand in hand with her strength of character. Despite the evil exposure that she endures at home from the negative presence of her mother, Tillie has an uncanny ability to separate her sentiments from her reality. For example, regardless of how badly her mother treated her after finding out about the Science fair, Tillie kindly pointed after seeing her mother's outfit for the Science fair that her mother looks and is very pretty. Similarly, she simply answers a submissive "Yes, mother", at the end, after her mother has killed the rabbit and still has the nerve to say "I hate the world." If anyone should hate the world it should be Tillie and yet she responds the way that she feels is most proper.

Yet, the most salient and obvious feature of Tillie's character is her intelligence. In Ruth's own words, the teachers have expressed that Tillie will one day be a "Marie Curie". Tillie internalizes the lessons in science with passion, as if Science and poetry were connected:

Atom. Atom. What a beautiful word.

Her natural curiosity (how long had she known about marigolds and gamma rays?) leads her to explore the world around her without fears, being led by who has become the most important person in her life: her teacher, Mr. Goodman. It is the love for learning, not necessarily her success at learning, what is more worthy of admiration in Tillie's character.

In all, Tillie has a lot of redeeming qualities that surpass the wrongdoings of her mother. She is a warrior led by her love for Science and learning. She is successful in spite of her sad upbringing.